Recently, in a car industry, environmental friendly technologies have been developed actively. For example, hybrid electric vehicles run by an electric motor and an engine (hereinafter, referred to as “HEV”) have been introduced in a market. An electric motor system for HEV has a high working voltage region such as several hundreds volts. Therefore, as capacitors used for such electric motor systems, metalized film capacitors having a high withstand voltage and excellent electric characteristics have been used.
FIG. 14A is a developed perspective view showing a configuration of a conventional metalized film capacitor of this kind; and FIG. 14B is a sectional view showing a part of a dielectric film thereof.
As shown in FIG. 14A, dielectric film 410 is made of, for example, polypropylene (PP) and has metal vapor-deposited electrode 411 on one principal surface thereof. Dielectric film 410 has non metal vapor-deposited portion 412, which is an exposed portion of dielectric film 410, in the longitudinal direction of one end in the width direction of dielectric film 410. With such a configuration, a metalized film is formed. A pair of such metalized films is wound in a state in which metal vapor-deposited electrodes 411 face each other with dielectric film 410 interposed therebetween, and metal sprayed electrode 413 is formed by thermal spraying zinc onto both edge faces.
Prior art information relating to the invention of this application includes, for example, Japanese Patent Unexamined Publications No. H4-163042 and No. 2005-93761.
However, in the above-mentioned conventional metalized film capacitor, the general capacitance range is from 0.1 to 50 μF, and the use temperature range is about −10° C. to +75° C. Meanwhile, the capacitance required for HEV is not less than several hundreds μF and the use temperature range is not less than from −40° C. to +90° C. The conventional capacitor cannot be used as it is for HEV.
Furthermore, since capacitance per unit element of a metalized film capacitor is about one hundred and several tens μF, in order to obtain about not less than several hundreds μF of capacitance, a plurality of metalized film capacitors may be connected in parallel. However, when the number of elements is increased, there are problems that cost rises and reliability is reduced.
If capacitance of one element can be considerably increased to be several hundreds μF, the number of elements can be reduced and metalized film capacitors for HEV can be produced stably and at a low cost.
Note here that capacitance C of the above-mentioned metalized film capacitor is expressed by the equation: C=∈×(S/d) (∈: dielectric constant, S: electrode area, and d: distance between electrodes).
When the thickness of a film as a dielectric substance is determined from the withstand voltage and the like, in order to increase the capacitance, it is necessary that an area of the electrode be increased, that is, the number of metalized films be increased. In this case, naturally, the shape of the element must be enlarged. When the shape of the element is enlarged, a displacement amount due to thermal expansion and contraction is increased. Furthermore, since the guaranteed temperature range in vehicle-mounted capacitor is wider than the general one by 45° C. or more, the displacement amount due to thermal expansion and contraction is further increased.
In general, a metalized film capacitor includes a metal sprayed electrode having relatively small thermal contraction and an organic matter based dielectric film having relatively large thermal contraction. Consequently, a portion in which the metal sprayed electrode and the film are brought into contact with each other is the most susceptible to thermal stress. Therefore, when the shape of the capacitor is enlarged and the use temperature range is further widened, the thermal stress is increased. As a result, the state in which the metal sprayed electrode and the film are brought into contact with each other is further deteriorated. Due to this deterioration, electrical loss (tan δ) that is one of the electrical characteristics of a capacitor may be deteriorated. In general, it is desirable that the increase in tan δ is not more than 50% with respect to the initial value.
Furthermore, in this kind of metalized film capacitors, in addition to increasing of a capacitance per unit element, there has been a strong demand for reducing the size of equipment or power supply unit to be used and making the capacitor have a small and thin size from a restriction such as an mounting area on a printed wiring board. That is to say, in order to achieve a large capacity and allow a capacitor element to be contained in limited space, the element must be further flattened.
FIG. 15 shows preliminary calculated volumetric efficiencies when capacitor elements having the same capacity are contained in each case having the same height and depth. FIG. 15 respectively shows a plurality of capacitor elements whose sectional shape is circular (S1), a plurality of capacitor elements whose sectional shape is elliptical (S2), one large elliptical shaped capacitor element obtained by enlarging and flattening the elliptical elements (S3), and two laminated elements obtained by winding an element with a large diameter and cutting it into two (S4). As is apparent from FIG. 15, it is shown that a large elliptical element shown by reference mark S3 exhibits the most excellent volumetric efficiency (97%). When only a volumetric efficiency of the element is considered, a laminated capacitor in which rectangular shaped films are laminated can provide a volumetric efficiency that is almost 100%. However, it is not easy to produce a large laminated capacitor by laminating thin films and cutting them. Furthermore, there is a problem as to withstand voltage and it is known that the intended withstand voltage per unit thickness cannot be achieved.
However, when the large elliptical capacitor element shown in S3 in the above is produced, a method of producing a large circular capacitor element and then flattening thereof is the most suitable to mass production. In this case, the strength of a winding core is important. When the thickness of materials of the winding core is thick and the strength is too strong, it is difficult to pull out a holding jig (not shown).
Furthermore, as shown in FIG. 16, in addition to the difficulty in flattening, there arises a problem that winding core 400C is returned to the original state after flattening, so that the capacitor element is swollen in the direction of 400D.
On the contrary, when the thickness of the material of the winding core is too thin and the strength is too weak, after the film is wound, a part of the winding core moves together with the winding core holding jig when the winding core holding jig is pulled out. As a result, as shown in FIG. 17, portion 400Q is protruded from end surface 400T. Furthermore, after processing, as shown in FIG. 18, for example, wrinkles 400W are generated in winding core 400C as the capacitor element returns to the original state in direction 400D. Thus, the capacitor element that has been processed in a flat shape becomes weak in strength and a desired shape cannot be maintained.
Furthermore, a metalized film capacitor as a single unit has a low humidity resistance property. Therefore, in order to dissolve this problem, a case mold type capacitor, in which a capacitor element is contained in a case and resin is cast into the case, has been developed and practically used. When a large capacity is required, a case mold type capacitor in which a plurality of capacitor elements are connected in parallel has been developed and practically used.